As a power source of a mobile device, or the like, a lithium ion secondary battery is mainly used. In recent years, the function of the mobile device or the like is diversified, resulting in increasing in power consumption thereof. Therefore, a lithium ion secondary battery is required to have an increased battery capacity and, simultaneously, to have an enhanced charge/discharge cycle characteristic.
Further, there is an increasing demand for a secondary battery with a high power and a large capacity as a power source for electric tools such as an electric drill and a hybrid automobile. In this field, conventionally, a lead secondary battery, a nickel-cadmium secondary battery, and a nickel-hydrogen secondary battery are mainly used. A small and light lithium ion secondary battery with high energy density is highly expected, and there is a demand for a lithium ion secondary battery excellent in large current load characteristics.
In particular, in applications for automobiles, such as battery electric vehicles (BEV) and hybrid electric vehicles (HEV), a long-term cycle characteristic over 10 years and a large current load characteristic for driving a high-power motor are mainly required, and a high volume energy density is also required for extending a cruising distance, which are severe as compared to mobile applications.
In the lithium ion secondary battery, generally, a lithium salt, such as lithium cobaltate, is used as a positive electrode active material, and a carbonaceous material, such as graphite, is used as a negative electrode active material.
Graphite is classified into natural graphite and artificial graphite. Among those, natural graphite is available at a low cost and has a high discharge capacity due to its high crystallinity. However, as natural graphite has a scale-like shape, if natural graphite is formed into a paste together with a binder and applied to a current collector, natural graphite is aligned in one direction. When a secondary battery provided with an electrode using natural graphite of high orientation property as a carbonaceous material is charged, the electrode expands only in one direction, which degrades the performance of the battery. The swelling of the electrode leads to the swelling of the battery, which may cause cracks in the negative electrode due to the swelling or may damage the substrates adjacent to the battery due to the detachment of a paste from the current collector. This has been an issue to be solved.
Natural graphite, which has been granulated and formed into a spherical shape, is proposed, however, the spherodized natural graphite is crushed to be aligned by pressure applied in the course of electrode production. Further, as the spherodized natural graphite expands and contracts, the electrolyte intrudes inside the particles of the natural graphite to cause a side reaction. Therefore, the electrode material made of such natural graphite is inferior in cycle characteristics, and it is difficult for the material to satisfy the requests such as a large current and an long-term cycle characteristic of a large battery. In order to solve those problems, Japanese Patent No. 3534391 (U.S. Pat. No. 6,632,569; Patent Document 1) proposes a method involving coating carbon on the surface of the natural graphite processed into a spherical shape. However, the material according to the method of the Patent Document 1 can address the issues related to a high capacity, a low current, and a medium-term cycle characteristics required for use in mobile devices but it is difficult for the material to satisfy the requirement for a large-size battery such as a large current and an ultra-long term cycle characteristics.
Regarding artificial graphite, there is exemplified a mesocarbon microsphere-graphitized article described in JP H04-190555 A (Patent Document 2) and the like. However, the article has a lower discharge capacity compared to a scale-like graphite and had a limited range of application. In the case of using a graphitized article obtained by the method of the Patent Document 2, it is difficult to achieve the cycle characteristic for a much longer period of time than the one for mobile applications, which is required for a large battery.
Artificial graphite typified by graphitized articles of petroleum, coal pitch, coke and the like is available at a relatively low cost. However, although a graphitized article of needle-shaped coke of high crystallinity shows a high discharge capacity, it is formed into a scale-like shape and is easy to be oriented in an electrode. In order to solve this problem, the method described in Japanese Patent No. 3361510 (Patent Document 3) proposes a negative electrode for a lithium ion secondary battery in which material a mixture of graphite particles and an organic binder is integrated with a current collector; and an aspect ratio of the graphite particle, a density of the mixture after the integration, and the size of a crystallite in the direction of c-axis, Lc(002), are defined in specific ranges. The negative electrode according to Patent Document 3 can allow the use of not only fine powder of an artificial graphite raw material but also fine powder of a natural graphite or the like, and exhibits very excellent performance for a negative electrode material for the mobile applications. However, its production method is cumbersome.
Further, negative electrode materials using so-called hard carbon and amorphous carbon described in JP H07-320740 A (U.S. Pat. No. 5,587,255; Patent Document 4) are excellent in a characteristic with respect to a large current and also have a relatively satisfactory cycle characteristic. However, the volume energy density of the negative electrode material is low and the price of the material is expensive, and thus, such negative electrode materials are only used for some special large batteries.
Japanese Patent No. 4738553 (U.S. Pat. No. 8,372,373; Patent Document 5) discloses artificial graphite being excellent in cycle characteristics but there was room for improvement on the energy density per volume.
JP 2001-23638 A (Patent Document 6) discloses an artificial graphite negative electrode produced from needle-shaped green coke. Although the electrode showed some improvement in an initial charge and discharge efficiency compared to an electrode of conventional artificial graphite, it was inferior in a discharge capacity compared to an electrode of a natural graphite material.
JP 2005-515957 A (WO 03/064560; Patent Document 7) discloses an artificial graphite negative electrode produced from cokes coated with petroleum pitch in a liquid phase. In the negative electrode, the electrode capacity density has remained as an issue to be solved. Also, the production involves an operation of using large quantities of organic solvent and evaporating it, which makes the production method cumbersome.
JP H09-157022 A (CA 2,192,429; Patent Document 8) discloses a graphite negative electrode using a graphite capable of occluding a larger amount of lithium ions and aiming to provide a high-energy lithium ion secondary battery, which electrode is obtained by subjecting a mixture of coke and silicon carbide as an initial material to high-temperature treatment and thermally dissociating silicon atoms. However, a pulverization process is needed since the graphite particles in the negative electrode are produced in aggregates, and the production method is cumbersome and takes high cost due to the pulverization loss. In addition, the pulverization process is accompanied by generation of lattice defects, and lithium ions irreversibly bond thereto, resulting in a problem of decrease in cycle characteristics.